In bacteriological research and development the sterile shake flask is used extensively. Commonly, a plug of cotton is used to keep the contents of the flask sterile and to permit the exchange of ambient air with the gases in the flask.
A major shortcoming of this very simple system is the slow rate of gas exchange through the cotton plug. Consequently, the gas exchange through the cotton plug is rate limiting rather than the biological process in the bacteriological medium. Elaborate sterile gas pumping systems have been developed and used to increase the rate of air through put. However, such systems are quite expensive, complicated to operate and maintain and provide a source of contamination.